The study considered the role that skin cells play in the overall sensation of touch, especially with regard to specialized skin cells known as Merkel cells. Sensations of gentle touch, such as a fine texture or the sensation of a light wind on the skin, are transmitted through nerve endings that frequently terminate at Merkel cells.
The question was whether Merkel cells or the nerve endings act as the sensors. According to the research team, the answer is...both. Merkel cells and the associated nerve endings appear to act as a dual-sensor system responding to light touch.
Four years ago, team members discovered a specific protein known as Piezo2 that serves as a so-called ion channel which is mechanically activated during the touching process. Ion channels are conduits that allow electrolytes (sodium, potassium, or other ions with positive charges) to initiate signals within nerves, eventually reaching the brain. These channels are found within the outer membrane areas of nerve fibers and certain cell types.
Piezo2 proteins were found to be made in neurons involved in touch sensing, including those with nerves that enter the skin and end adjacent to Merkel cells. It is assumed that the ion channels are activated by literal micro-stretching of the nerve membrane in which they reside-from light pressure against the skin transmitted through to the ion channels.
The research team was able to genetically alter mice to simultaneously produce protein markers when the Piezo2 protein was activated, one of which was a fluorescent protein known as GFP. These markers showed high levels of Piezo2 in the mice's Merkel cells, thus providing a reference for the next step.
In a particularly adept bit of genetic engineering, the team was able to produce a strain of mice that had the Piezo2 gene removed from all skin cells, with Merkel cells included, but only from skin cells. In every other area that Piezo2 was produced in these mice, it remained intact.
These mice responded normally to general sensitivity testing for touch and pain. However, in a test where thin fibers are used to press against the paws of the mice with differing amounts of force, the mice did not respond to gentle pressure compared to the control mice with intact levels of Piezo2 in their skin cells.
Collaborative efforts at Columbia University and the Medical College of Wisconsin gave complementary results with decreased sensitivity to touch when Piezo2 was not present in the Merkel cells. The conclusion is that Merkel cells have an important role in the sensory input of touch, especially with gentle or fine touches, and that Piezo2 is required for the mechanism.
By better understanding this type of sensory input, the team hopes to understand various pain mechanisms that are related to touch. Presumably these mechanisms will transfer to humans and this research can lead to better pain management and alleviation for touch-related pain syndromes.